Tar sands or oil sands such as are found in Canada, contain vast reserves of hydrocarbon resources of the type referred to as heavy oil or bitumen. Such heavy oil or bitumen is a hydrocarbon that has a high specific gravity and viscosity. These properties make it difficult to extract the hydrocarbon from the tightly packed sand formations in which it is found because unlike lighter oil deposits, heavy oil and bitumen do not readily flow at in situ conditions.
In prior Canadian Patent No. 2,299,790, a condensing solvent based in situ hydrocarbon recovery process is disclosed. This patent teaches, among other things, using a condensing solvent and controlling the in situ pressure to achieve a condensation temperature for the solvent within the formation which is suitable for reducing a viscosity of the in situ hydrocarbon by warming and solvent effects so that the hydrocarbon will flow under the influence of gravity. The result of this process is a volume in the formation which is stripped of the mobilized hydrocarbons, and which is called a gravity drainage chamber. As more solvent is circulated more hydrocarbon is removed resulting in a chamber which grows upwardly and outwardly from the injection well.
Canadian Patent No. 2,351,148 teaches, among other things, using a solvent which has been purified sufficiently to allow the solvent to achieve bubble point conditions at the extraction interface of the gravity drainage chamber whereby non-condensable gases naturally arising from the warming bitumen or hydrocarbon will be carried away with the draining liquids also in liquid form. In this way, a continuous extraction process is achieved at the extraction interface, because the potential impediment of an insulating layer of non-condensable gases existing between the incoming condensing solvent and the extraction interface is removed as part of the process.
The geological characteristics of the tar sands or oil sands can vary from deposit to deposit. While some deposits are relatively thick deposits in the order of 40 to 50 or more meters thick, many deposits are relatively thin being less than 20 meters thick and in many cases even 10 meters or less thick. In addition, the characteristics of the overburden can vary considerably. In some cases, the overburden is comprised of the cap rock which can act as a containment layer, but in other cases the overburden may be a sand layer or gravel or other porous material that provides poor confinement.
Where good confinement is available it is preferred to let the chamber grow to all the way to the overburden layer to extract all of the available hydrocarbon, but, leaving the overburden exposed to condensing solvent in the chamber is undesirable. More specifically, the overburden will continue to attract condensing solvent and the latent heat of condensation of such condensing solvent will be passed to the overburden but to no useful extraction effect. There is simply no hydrocarbon located in the overburden which can be warmed and removed. Therefore, any heat transfer to the overburden layer is wasted, thereby reducing the efficiency of the condensing solvent process.
In some cases, the overburden layer may not be a good confinement layer. In cases where the overburden layer is sand or other porous material it may also be saturated with water. In such a case, if the chamber growth extends vertically to the overburden layer the water will be provided with a pathway into the chamber which could result in the chamber being water flooded. Once the chamber is water flooded, further extraction from the chamber through a condensing solvent process is unlikely. Thus, when poor confinement exists it is preferred to stop vertical chamber growth at a point below the overburden layer to preserve a layer of hydrocarbon to that provides the necessary confinement.